1. Field of the Invention
This invention relates to a scanning optical apparatus for scanning a light beam from a light source on a scan surface through a deflector and a lens system, and in particular to a scanning optical apparatus provided with a mechanism for detecting and correcting the focus position deviation of the imaged spot of a light beam on the scan surface which is attributable to the fluctuation of the environmental conditions such as temperature, etc.
2. Related Background Art
In recent years, as scanning optical apparatuses, use has widely been made of laser beam printers in which a laser source is modulated in conformity with an image signal and a laser light from said modulated laser source is periodically deflected by a deflector and is converged into a spot on a scanning surface, e.g. a photosensitive recording medium, by a lens system, and is exposure-scanned to thereby accomplish image recording.
Now, the conventional laser beam printers have suffered from the disadvantage that each member constituting the lens system is thermally deformed by a change in the environmental temperature and the converged position of the laser beam on the photosensitive member (the scan surface) deviates to deteriorate the quality of image.
As a means for overcoming such disadvantage, there is disclosed in Japanese Laid-Open Pat. application No. 60-100113 a laser beam printer provided with focus deviation detecting means for receiving a part of a beam scanned on a photosensitive member and detecting focus deviation, and a lens moving device having a correction lens for correcting said focus deviation.
However, in the aforementioned Japanese Laid-Open Patent application No. 60-100113, an astigmatism system is employed as the focus deviation detecting system. This system is effective for a stationary light beam, but is unsuitable for the detection of the focus deviation of a light beam scanned at a high speed.
Also, in Japanese Laid-Open Patent application No. 62-81873, there is shown a system in which a plurality of light receiving elements arranged in a direction perpendicular to the direction of scanning of a light beam are used as focus deviation detecting means and the number of the light receiving elements which have received the light beam is used as information for focus adjustment.
However, in the system shown in this Japanese Laid-Open patent application, the size of the diameter of the imaged spot in the sub-scanning direction, which is a direction, perpendicular to the scanning direction, can be detected, but the size of the diameter of the imaged spot in the main scanning direction, which is, in the scanning direction cannot be detected. In, a laser beam printer, the size of the diameter of the imaged spot in the main scanning direction is usually smaller than the size of the diameter of the imaged spot in the sub-scanning direction and therefore, the imaged spot in the main scanning direction is more liable to be affected by the fluctuation of the environmental conditions and greatly changed than the imaged spot in the sub-scanning direction. Consequently, the above-described system cannot provide a scanning optical apparatus which prevents the deviation of the imaged spot attributable to the fluctuation of the environmental temperature and is capable of accurately scanning a laser beam. Further, in the system shown in the above-mentioned publication, it is necessary that a predetermined number of bits of information be memorized in advance in the apparatus, and if the memory of the predetermined number of bits of information is lost due to some trouble or other, detection of the focus position deviation becomes impossible.